Various rubber compositions, particularly rubber compositions containing diene-based elastomers, usually contain particulate reinforcing fillers to promote enhanced physical properties of the rubber composition. Such reinforcing fillers include, for example, one or more of rubber reinforcing carbon black and amorphous synthetic silica such as, for example, precipitated silica. Such rubber reinforcing fillers and their use in reinforcing elastomers, in general, are known to those having skill in such art.
Exfoliated graphene platelets are platelets which have been exfoliated from intercalated graphite layered planes.
Exfoliated graphene platelets have been suggested as being useful to aid in promoting one or more of electrical, thermal and mechanical properties for various polymers such as thermoplastic or thermoset polymers. Graphene platelets have been suggested for use in rubber compositions for tire components. For example, and not intended to be limiting, see U.S. Pat. Nos. 7,479,516, 7,224,407 and 6,892,774 and Patent Publication No. 2006/0229404.
In the above mentioned U.S. Pat. No. 6,892,774, rubber compositions containing diene-based elastomer(s) rubber with expanded graphite have been suggested, particularly for tire treads for ice traction, while maintaining abrasion resistance. The expandable graphite is said to have a particle size of from 30 to 600 micrometers. The expandable graphite is said to be composed of sheets formed from graphite stacked in layers and as being a known material and produced by a known method which may comprised of, for example, immersing graphite particles in a strong acid (e.g. sulfuric acid) and oxidizing substance (e.g. nitric acid) followed by an intercalation treatment to insert the acid between layers of the graphite particles to thereby provide expandability of the graphite. The expandable graphite is then expanded by vaporizing the interlayer compound (e.g. at a temperature of about 300° C., or possibly even as low as 160° C. in the rubber composition). Apparently expandable graphite having an expansion initiating temperature of 190° C., or less, are said to be commercially available.
In the above mentioned U.S. Patent Publication No. 2006/0229404, rubber compositions containing diene-based elastomer(s) have been suggested containing an exfoliated graphite having been intercalated with an elastomer. For such intercalating method, the graphite is intercalated in a manner similar to the above method with acid and oxidizing material and then expanded, or exfoliated, by sudden exposure to high heat following which the expanded graphite is mixed with suitable monomers which are polymerized to form an elastomer matrix which contains the exfoliated graphite which, in turn, may be used to form a component of a tire.
For use in this invention, exfoliated graphene platelets may be formed by intercalation of natural graphite with its layered planes followed by exfoliation of the intercalated graphite. Intercalation of the graphite layered planes may be accomplished, for example, by immersing the natural graphite in a mixture of high concentration of strong acid (e.g. sulfuric acid) together with an oxidizing agent (e.g. potassium permanganate or nitric acid) followed by an intercalation treatment to introduce the acid and oxidizing agent into the galleries between the layered planes of the graphite to form an interlayered product and thereby intercalate the graphite layers (e.g. the galleries between the layers). As a part of the intercalation process, the galleries may be expanded to thereby increase the distance between the plates.
The layers, or planes, of the intercalated graphite galleries may then be physically or chemically exfoliated, or separated, to form individual graphene platelets by, for example, thermal expansion, or ultrasonication followed by a reduction using hyrazine in a water suspension.
For this invention, it is proposed to evaluate use of nano-sized exfoliated graphene platelets in a rubber composition.
For such evaluation, it is desired for the somewhat irregular shaped exfoliated graphene platelets to be nano-sized in a sense that they have an average thickness in a range of from about 1 nm to about 5 nm and an average lateral dimension in a range of from about 0.1 micrometer to about 1 micrometer (e.g. about 0.01 to about 1 square micrometers), desirably in a range of from 0.1 micrometer to about 0.5 micrometer (e.g. about 0.01 to about 0.25 square micrometers).
It is envisioned that such nano-sized exfoliated graphene platelets with, for example, an average lateral dimension in a range of from about 0.1 to 1 micrometer have an average surface area per gram in a range of from about 20 to about 800 m2/g (square meter per gram).
To achieve such nano-sized exfoliated graphene platelets, intercalation of the graphite may be conducted, for example, by extended oxidation such as, for example, by exposing the graphite to the aforesaid oxidation process for a sufficient period of time cause the planes to be broken up, or fractured, into small particle sizes to promote a creation of the nano-scale, or nano-sized, exfoliated graphene platelets.
As previously, indicated, an important aspect of this evaluation is the use of the aforesaid exfoliated graphene platelet with nano-scale thickness and submicro-scale lateral dimension size to promote exfoliated graphene platelet dispersion and various rubber physical properties for the rubber composition as well as to promote electrical conductivity for the rubber compositions used in various tire components. As previously indicted, an average lateral dimension of the nano-scale exfoliated graphene platelets is preferably less than 1 micrometer, and desirably less than 0.5 micrometer to promote the best graphene platelet/rubber dispersion and graphene platelet/rubber interaction to promote mechanical reinforcement of rubber compositions for tire components, as well as for an improved electrical conductivity of the rubber composition. It is believed that such observation and development is a departure from past experience for tire components which contain exfoliated graphene platelet reinforcement using significant larger graphene platelets.
The electrical conductivity of exfoliated graphene platelets in rubber compositions is highly anisotropic. The in-plane electrical conductivity of the graphene platelets is very high, in the range of 104 to 106 S/cm. However, the through-plane electrical conductivity of the graphene platelets is very low, at the range of 0.01 to 1 S/cm.
Therefore, the electrical conductivity of the rubber composition is much higher when exfoliated graphene platelets are primarily dispersed randomly in the rubber composition due to non-varnishing electrical conductivity which thereby renders the rubber composition containing randomly dispersed graphene platelets to be relatively electrically conductive due to non-varnishing electrical conductivity mechanism.
However a dispersion of the graphene platelets in a rubber composition is electrically non-conductive when a significant portion of the platelets are oriented parallel to each other in the rubber composition which thereby renders the rubber composition containing the oriented graphene platelet to be electrically non-conductive due to poor through-plane electrical conductivity of the graphene platelets.
This phenomenon is significant and important.
When the large exfoliated graphene platelets are of a relatively large lateral size (e.g. an average lateral dimension greater than 1 micrometers, especially larger than 5 micrometers) they tend to be oriented in a direction aligned with the platelets plains when the rubber composition is processed in the processing direction such as by, for example, milling and calendering, and such alignment thereby promotes electrical conductivity for the rubber composition in a direction parallel to the graphene platelet alignment. However, the rubber composition typically exhibits a rather poor electrical conductivity in a direction perpendicular to the graphene platelet alignment.
However, the very small nano-sized graphene platelets for this invention (e.g. 0.1 to 1 micrometer, and particularly 0.1 to 0.5, micrometer lateral dimension) have been observed to more randomly disperse in different plane directions in a rubber composition upon processing the rubber composition which is an important phenomenon to promote high electrical conductivity for the rubber composition which is not limited to the processing direction of the rubber composition.
In the description of this invention, the term “phr” is used to designate parts by weight of a material per 100 parts by weight of elastomer. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” may be used interchangeably, as well as “unvulcanized” or “uncured”, unless otherwise indicated.